Radio-frequency (RF) mobile communication devices use digital hardware and digital signal processing techniques at baseband in conjunction with analog circuitry to condition a signal for transmission via an antenna. The final stage of conditioning prior to transmission involves amplifying the signal using a power amplifier.
It is desirable to operate the power amplifier near saturation in order to attain high power efficiency. However, efficiency in a power amplifier is achieved with the penalty of an inherently non-linear signal transfer characteristic, which is typically a complex function involving temperature dependence.
Pre-distortion has been used which, in its simplest form, involves distorting the transmission signal prior to amplification, using a polynomial function that is the inverse to the distortion introduced by the power amplifier, thereby linearizing the behavior of the power amplifier. In order to create a pre-distortion model for inverting the nonlinearity introduced by the power amplifier, an iterative method is used where the input to and output from the power amplifier are repeatedly passed through an estimator, and statistical methods are used to perform linear and nonlinear regression on the signal data.
However, since mobile communication devices operate under different radio power output conditions, non-linear distortion of the transmission signal by the power amplifier may change when there is a power step. This can result in spectrum re-growth, which diverts some of the energy from a desired frequency channel into adjacent frequency channels. This, in turn, results in a loss of performance within a desired frequency channel as well as the creation of interference within adjacent frequency channels.
In order to compensate for each of these nonlinear profiles, a slightly different polynomial curve must be used. Therefore, prior art systems require either pre-characterization of the device to create pre-distorting look-up tables (LUTs), or the use of real time polynomial estimators.
According to the LUT approach, the nonlinearity is pre-characterized during factory calibration of the device such that different polynomial coefficients are stored in the LUT for each power operating point. The different coefficients are instantly fetched based on the output power requirements. LUTs provide stable pre-distortion but suffer from very slow convergence time as each amplitude bin is individually trained, as well as suffering from increased memory and computational requirements for storage and fetching of the coefficients from the LUT. Also, the LUT does not work correctly in the event the response of the amplifier deviates from the pre-characterized coefficient tables.
Real time polynomial estimators converge rapidly but suffer from instability. This is because a small change in polynomial coefficients can lead to a large change in the behavior of the polynomial. In the initial phase during which the system is converging, the radio transmitter will generate significant nonlinear distortion that may violate spectral mask requirements and exceed the maximum acceptable bit error rate. The perturbation generated in the initial phase of the estimation can be so strong that it leads to instability of the entire estimation process. One solution to the instability problem is to let the pre-distortion system train to the new power level before transmitting the signal. However, this wastes valuable transmission time and compromises the overall data throughput of the communication system.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of protection is thereby intended.